It is known to be desirable at times to be able to machine a thread, either internal or external, with a reliably oriented start position, or rather, with a reliably oriented finish position, so that when two components are screwed together to a desired torque, one has a predetermined orientation with respect to the other.
The predetermined orientation may permit functionality of one or both of the components. In internal combustion engines for motor vehicles, for example, a spark plug is screwed into a bore associated with a cylinder in the engine so that the spark plug may ignite fuel injected into the cylinder. Spark plugs are well known and usually comprise a central live electrode mounted in an insulating ceramic body surrounding the live electrode and which itself is mounted within an annular ground electrode. The annular ground electrode is completed by an arm from the edge of the annular part that overhangs the central electrode to form a predetermined air gap between the ground electrode and the live electrode. A spark forms across the spark plug gap when a potential difference is applied to the electrodes.
Modern engines have fuel injectors that inject fuel at the last possible moment before ignition. The injected fuel may be arranged in directed sprays or jets that pass close to the spark plug gap. However, such sprays must not be allowed to spray between the gap or else they can disrupt the development of the spark. If this were to happen, the spark would not be as hot as intended and this would lead to inefficient and or incomplete burning of the fuel. This would most likely mean that the engine would fail to meet emissions targets. The annular ground electrode is provided with a thread to enable the plug to be screwed into a corresponding thread in the engine cylinder head. The cylinder head has a spot face, being the flange of the cylinder head surrounding the threaded bore that receives the sparkplug and against which spot face a seal washer on the sparkplug seats after tightening to a predetermined torque.
When a spark plug is screwed into a cylinder head, it has hitherto been the case that the rotational position of the ground electrode arm is entirely arbitrary. This is not satisfactory in modern engines where the spray of fuel from the injectors is both late, coinciding with the development of the spark, and close to the spark plug electrodes. If the spray should hit the arm, the spray may be deflected between the electrodes and/or displace gas between the electrodes which is ionised as a precursor for the electric arc that follows shortly and provides the requisite heat to ignite the fuel. Ideally, the spark plug arm should connect with the edge of the annular electrode diametrically opposite to opening of the fuel injectors or at most be within ±45° of that position, so that there is no opportunity for the arm to disrupt the spray from the injectors. Indeed, an alternative position directly opposite would also be perfectly acceptable, but the permitted error in the angular position before spray disruption was possible would be much less. Spray disruption also leads to potential wetting of the spark plug electrode that results in carbon deposits after combustion which can later cause misfires due to hot spots.
For any given threaded bore and spot face, sparkplug manufacturers (for example NGK Spark Plugs Mfg. (U.S.A.), Inc.) can provide sparkplugs where the arm will subtend the same angle with respect to a given angular position of the spot face with great reliability and within an error of less than ±25° of arc. This is achieved through an accurate placement of the arm with respect to the thread on the plug. However, providing a spark plug with this accuracy of thread position is only half the requirement, because if the thread in the bore of cylinder head does not itself have an accurate angular orientation of its start position, the accuracy of the thread on the plug is to no avail. Indeed, if the required position of the arm at a zero position is ±45° of arc, the positional error of the thread in the cylinder can be a maximum of ±20° of arc if the error in the plug is ±25°.
Thus the problem addressed by the present invention, although it is to be understood that the invention is not limited to its application to engines, is to provide a method of forming threads in engine cylinder heads for receipt of spark plugs where the start position of the thread with respect to a specific zero angle on the spot face is reliably determined and repeatably produced. In fact, such a problem is easily achieved with current methods, but not within the confines of the exacting requirements of mass production.
The art of producing internal threads is well explored and there are numerous methods that fall in three basic categories of cutting, forming and milling. Both cutting and forming may be considered to fall within another broad category of tapping, as both require a tool to be driven into the workpiece at an axial rate of one thread pitch per rotation. Thread forming produces a stronger thread than cut tapping or thread milling because instead of removing material, thread forming flows the grain of the metal into the desired geometry. However, milling threads with a thread miller is a much easier process to control numerically. A thread milling tool (of diameter smaller than the bore to be threaded in the case of internal bores) is rotated at high speed about its own axis, moved in a helical path corresponding with the thread to be cut and its start position is always precisely known, largely because it is essential to know before a thread can be cut.
However, returning to the specific issue of engine cylinder head manufacture and threading, modern cylinder heads often have substantial depth or volume and in many instances the bore for receipt of a spark plug will itself be at the bottom of a deep recess. The bore to be threaded is obviously easily available from underneath the cylinder head, from within the confines of the putative combustion chamber, but since the surface against which the start position of the thread to be formed must be oriented is the spot face on the outside of the cylinder head, error is introduced immediately by the multiple changes in orientation of the workpiece and change of tool required. That is to say, the machining operation needs to keep precision with: forming the spot face by machining from one side of the bore (outside of the cylinder head); rotating the workpiece to approach the bore from its other side; replacing the spot forming tool with a thread-forming tool. The inaccuracies introduced by having to rotate the part result in a less accurately orientated thread than is possible if both the thread and the spot face are machined from the same side of the part. Actually, in the case of thread cutting, the tools can be the same. That is, a thread cutting tool bit can be formed on the end of a spot face milling tool. Thus more precision can be achieved by not having to rotate the workpiece or change tools but instead use a tool which first cuts the spot face and second forms the thread, both from the same direction. Since the two features are machined by the same tool, the inaccuracies are greatly reduced to the manufacturing and setting tolerances of the tool and the positional accuracy of the machine.
This is generally possible, and even possible at the bottom of a deep recess, but there are two problems that are not overcome. The first is that the thread is a cut thread, and this is less desirable. The second is that, where the bore is at the bottom of a deep recess, the cutting tool forming the thread is necessarily on the end of a long tool that as a result introduces flexibility. The “cylindricity” (the degree to which the peaks and troughs of the cut thread lie on right circular cylindrical surfaces) cannot be guaranteed, at least not with only one pass of the cutting tool.
Thread forming is preferred and this uses a tool which is simply screwed into the bore with the same helical motion that the final thread is to have. The tool has the desired end thread formed on it, generally with points and flats so that the thread forming parts of the tool are restricted primarily to the points. The thread is formed entirely by deformation of the bore wall as the tool is progressed through the bore. There is no cutting (in the sense of material removal), from the bore. Moreover, the start position is equally easy, in principle, to assure; it is simply a matter of engaging the spot face with the tool so that the zero angular position desired for the thread start coincides, angularly, with the start of the thread on the forming tool.
However, CNC machining centres are not easily programmed to achieve a particular angular position of a tool at a particular position. Instead they are controlled to rotate and move at particular speeds and to move in particular directions. A tool moves in the x, y, z co-ordinate space, and rotates about the z-axis which is the axis of a tool holder. The tool holder certainly has a zero or start angular position, as well as a zero start position in the x, y, z co-ordinate space. Tool bits fit into the tool holder in a unique angular orientation. However, to provide a tool bit where the length of the tool and its angular position are both guaranteed with great precision, sufficient to determine the start position of the thread form, is excessively difficult and expensive to achieve. One option is to be unconcerned about such aspects and instead to simply programme the CNC machine to cut a trial thread in a first workpiece, determine where the thread starts, and then adjust the operation of the CNC machine so that in subsequent workpieces the thread is formed in the correct orientation. However, this is inefficient and creates unnecessary scrap. With the number of CNC machines used in large scale manufacturing and the low number of operators available to operate them it is also too time consuming to do this for each machine individually. Further to this, if it were to be done manually by an operator then this introduces an opportunity for human error in not setting the tool or machine correctly. This would lead to unacceptable levels of scrap parts and broken tools along with a significant amount of machine downtime which is very expensive to a large scale manufacturing business.
It is an object of the present invention to provide a method of reliably forming an angularly oriented thread on or in a workpiece using a CNC machining centre, such as in an engine cylinder head for receipt of spark plugs, in a way which guarantees that the first part produced is within specification without special operator interaction.